The invention concerns a slipping friction clutch which has at least one pair of wet-running friction areas and especially has a slipping converter bridging clutch for the hydraulic torque converter of a motor vehicle automatic transmission.
Hydrodynamic torque converters with a converter bridging clutch for an automatic transmissions of motor vehicles are known in many ways. Thus DE A 199 09 349 describes a hydrodynamic torque converter of this type, which has a pump impeller that is connected from the drive-side through a converter housing with a drive shaft, a turbine wheel can be connected with a drive shaft and a guide wheel, whereby the wheels together form a converter circuit filled with hydraulic fluid. A converter bridging clutch which has an annular disk-like bridging subassembly pressing on an interior wall of the converter housing by means of hydraulic pressure arranged between the converter housing and the turbine wheel is provided in the torque converter, as is a space constructed between the converter bridging clutch and the converter housing which is connected with an axial borehole of the output shaft whereby a connection for a converter circuit is provided on the side of the converter bridging clutch facing away from the converter housing.
These hydrodynamic torque converters find a wide use in motor vehicles and in particular in passenger vehicles for reasons of comfort. In order to avoid energy losses during an operating phases which require no shifting of a gearbox connected with the torque converter that are conditioned by the slippage between the pump impeller and the turbine wheel, these torque converters are provided with the bridging clutch. The bridging subassembly of this bridging clutch, acting as a piston in a certain way, directly takes over the transmission of torque between the converter housing and the output shaft in the state where it is pressed on the converter housing. Moreover, the friction area between the bridging subassembly and the converter housing is cooled by the hydraulic fluid flow which, for example, flows through intermediate spaces between the converter housing and the bridging subassembly pressed onto the converter housing into the space between the bridging subassembly and the converter housing, and from there into an axial bore hole of the output shaft.
Operating converter bridging clutches with slippage is known, whereby this slippage can arise, according to the design of the power train and/or as a function of the gear steps fed in and/or as a function of the operating state of the drive interacting with the flow converter either for a short time, for example during shifting processes, or as a continuous slippage through the entire operating range of the flow converter. During the slippage phase, a power loss in the region of the friction lining or friction areas results in the form of heat which can be very high under certain operating circumstances. Such operating circumstances occur, for example, during mountain driving with trailers where a high power loss can result over a longer time and in changing from the unbridged to the bridged state of the converter clutch where owing to the times of high slippage in a short interval of time, a very high power loss or amount of heat can arise.
Therefore, hydrodynamic torque converters for the automatic transmission of motor vehicles have already been proposed, where measures were taken to generate an oil flow reducing the thermal load of the converter bridging clutch. Thus EP 78651 describes a flow converter with a bridging clutch in which channels are provided on the side of the rotary piston facing away from the friction lining or the friction area, which are connected through openings with the first chamber formed axially between a radial partition of the housing and the rotary piston, on one hand, and with the second chamber accommodating the turbine wheel and the pump impeller, on the other. Oil flows through the channels from the second chamber into the first chamber which serves to cool the viscous coupling formed in the torque flow between the rotary piston and the turbine hub.
The oil flow generated in this connection nonetheless rises due to the fact that the torque which can be transmitted by the bridging clutch is diminished as a consequence of dynamic or kinetic processes occurring in the oil flow. The torque transmission capacity of the bridging clutch moreover diminishes with increasing rotational speed as well as with rising volume flow.
A further disadvantage can be seen by the fact that the oil flow is dependent upon temperature, and therewith on the viscosity of the oil, as well as on the pressure difference between the pressures existing on both sides of the converter piston.
Hydrodynamic torque converters with bridging clutches have already been proposed in which the friction linings are provided with grooves for conducting oil and cooling. EP 0 428 248 describes a hydrodynamic converter whose converter bridging clutch has an annular friction lining in which channels are provided to permit the oil from the pressure chamber to flow over the annular friction lining and, in this way, to remove heat from the friction lining if this continuously runs during slippage on the stationary cover.
DE A 44 20 959 describes a hydrodynamic flow converter with a converter bridging clutch, whereby channels are provided in the radial region of the friction areas in at least one of the subassemblies, supporting or forming the friction areas, which in an axial position of the friction areas enable an oil flow from the one pressure chamber through the channels radially to the axis of rotation of the flow converter. The linear dimensioning and the shape of these channels or stampings moreover must take place such that the flow resistance arising is extended to the critical performance of the torque converter or the converter bridging clutch. This means that even at the maximum oil temperature possible, only so much oil may run off from the second chamber into the first chamber that the systemic pressure in the torque converter does not collapse.
Common to the two latter solutions is that the channels or grooves produce a direct hydraulic connection between the two pressure chambers in one of the friction linings. Whereby large tolerances in the channel depth arise according to the manufacturing process and this leads to a strong dependency of the oil flow through the depth of the channels. Furthermore, these channels frequently wear out so that a major provisioning of the channels must take place, owing to which the wet-running clutch accommodates an excessively high volume flow in the new state which a typical motor vehicle transmission cannot deliver.
Furthermore, channels constructed as blind grooves without through flow are known which, nevertheless, do not enable any cooling of the lining.
Furthermore, brake disks and clutches have become known which have depressions for self-induced cooling by air or oil, but these do not seal off the outer edge radially against the radially inner edge so that compression cannot take place by means of the coolant fluid.
Channels constructed as grooves in this manner could only be used with very expensive three channel converters whereby in these cases, the coolant oil, however, is not, at the same time, a pressure oil.
The object of the present invention is to create a wet slipping friction connection, especially for the converter bridging clutch of a hydrodynamic torque converter, whereby a cooling oil flow rising with the slippage rotational speed guarantees a sufficient cooling independently of the surface pressure of the bearing surfaces, and which is simple in construction and cheap to manufacture.
The invention, therefore, provides a slipping friction clutch which contains at least one pair of friction areas running in oil, whereby the pair of friction areas can be pressed against each other through an oil pressure difference hydraulically controlled from outside on a pressure piston under a differential rotation speed such that this differential rotational speed can be diminished over a specified period of time by the friction clutch, or can also be continuously regulated in a stationary manner. For this purpose, in the state acted upon by pressure, the friction area pair seals off the high pressure chamber, which lies radially outward, against the low pressure chamber, which again lies radially inward, whereby oil-conducting depressions are created in a friction area, which stands in direct hydraulic connection with the radially inward lying low pressure chamber. The depressions form channels which begin at the radially inside pressure chamber and which lead back, peripherally offset, to the same interior pressure chamber. These depressions have a course such that, during slippage of the clutch, the dragging action of the friction area sliding past in the peripheral direction along the depressions. That is, the channels have a component along the depressions so that an oil flow through, which rises with the slippage rotational speed, is induced and then cools the friction area.
Advantageously, cooler oil flows through one or more openings in the pressure piston, out of the high pressure chamber into the low oil pressure chamber lying inside the friction area pair, such that it flows directly into the region of the inlets and outlets of the depressions where it is mixed with the hot oil and is pumped through the depressions through the drag effect mentioned.
The friction clutch, according to the invention, offers the advantage of a slippage-dependent friction area cooling adapted to needs as it is especially necessary for converter bridging clutches. The oil flow through the converter is moreover adjustable independently of the depressions constructed as cooling grooves, for example, through baffles. The diminished oil flow through in the new state in comparison with the usual short circuit grooves with their provision on the basis of wear and tear and tolerances leads to the transmission having to furnish less oil. No counter pressure build up takes place in the grooves and therewith no diminution in the transmission capacity. The friction clutch can be simply and cheaply manufactured without narrow tolerance requirements. The friction clutch offers a neutral behavior in relation to push and pull whereby the grooves for reducing the drag factor, during pressure reversal, act as blind grooves which bring about a tug between friction lining and friction area which acts as a lifting off pressure.